In a transmitting/receiving system in which data is transmitted and received by using application software sent through a network, because application software is placed at the client end, there is a possibility of unauthorized execution of the application software at the client end by methods such as providing a pseudo server.
An example of a method that has been conventionally known as a typical method for protecting such application software from unauthorized execution is a method in which a handshake between a server and a client is performed via network communications so that the server can recognize the program execution status at the client end point by point, and the application software is made available only during a certain period of time from a point in time at which the client receives a permission signal to use the application software from the server. A method for protecting application software from unauthorized use by dividing data to be transmitted and received so as to obfuscate the data has been also known.